1. Field of the Invention
The present invention relates generally to the control of gaseous fuel burners as used in various heating, cooling and cooking appliances. In particular, the present invention relates to a method and apparatus for setting and maintaining the proportions of fuel gas to air in the combustible mixture supplied to a power or induced draft, preferably premixed, burner at a desired firing rate.
2. The Prior Art
In the field of gas burner technology, relating to burners such as may be used in furnaces, water heaters, boilers, and the like, it is desirable to control the operation of a burner beyond merely supplying gas and providing air for combustion at a fixed flow rate, and igniting the mixture. Numerous factors must be considered in the construction, placement and operating conditions for a gas burner.
Some prior art appliances provide a fixed air supply to a burner, and must, therefore, not only supply enough air to prevent excessive production of carbon monoxide and oxides of nitrogen under ideal operating conditions, but also must provide a safety margin to account for incidences such as a blocked vent or an overfire condition (i.e., a significant increase in the firing rate above the rated value). Therefore, a standard appliance is typically designed with an excess air level significantly higher than would be required if changes in firing rate or air flow could be compensated for automatically. The additional safety margin of excess air can result in a significant reduction in appliance efficiency. Accordingly, it would be desirable to more closely control the fuel to air ratio.
In certain environments, in which human safety is a consideration, a burner must be operated in such a manner as to avoid the production of certain gases (such as carbon monoxide or oxides of nitrogen), beyond certain defined limits. The provision of air in excess of the applicable stoichiometric ratio for combustion of the particular fuel gas being burned may help to ensure safe operation and burning conditions, but may also create an inefficient operating situation.
Gas burner designs are being made in which the supplies of fuel gas, primary combustion air and secondary combustion air (if such is supplied) are capable of being closely physically controlled in finite increments. It is desirable to provide a method of monitoring the operation of the burner so that the incremental control of the gas and air supplies can be used to the best advantage to facilitate safe, and efficient operation.
It is, in general, a true proposition that a burner, which operates closely to stoichiometric conditions is more efficient than a furnace which is operating, for example, with a large amount of excess air. If the fuel gas is known, and the flow rates of fuel gas and combustion air are known, the actual combustion conditions, relative to stoichiometry are defined.
It is presently becoming popular in the art to provide appliances which have the capability to modulate or vary the fuel flow over a wide range, thus making a wide range of heating capacity (firing rates) available with a single appliance. Modulating capabilities can greatly increase a system's overall efficiency. Two-stage systems, i.e., systems capable of operating at two firing rate levels, are available, but are limited in their scope and range of operation due to their typical inability to precisely control the fuel gas and air mixture, and the need for a wide excess-air safety margin. A continuously modulating appliance, to be effective and efficient, would require close control of the fuel to air ratio. Though it is possible to directly measure the fuel and air flow rates independently and thereby determine the fuel and air mixture, such a detection system would require expensive sensor systems and be complex and possibly overly costly for most appliance applications of interest.
One method of monitoring burner operation, toward controlling same is disclosed in Noir et al., U.S. Pat. No. 4,188,172. In the Noir et al. '172 patent, a control burner is connected in parallel, with regard to the fuel gas and combustion air lines, with a main burner. The control burner is connected to two control loops. The first control loop consists of a waterfiller calorimeter, which surrounds the control burner. This calorimeter is used to determine the heating value of the fuel. The fuel flow is then adjusted to maintain a constant heat flux in the main burner. The second control loop consists of a temperature sensor located at the tip of the control burner flame. The control system of that reference functions on the basis that for a given firing rate, the flame temperature, for example, at the tip, will attain a maximum temperature, when the fuel/air ratio is at or near the theoretical stoichiometric ratio for the particular fuel. The air flow to the control burner is then varied until a peak temperature is reached. Then the air flow to the main burner is set at a predetermined multiple so as to achieve a desired fuel/air ratio in the main burner.
The control system of the Noir et al. '172 reference requires substantial calibration, as well as the provision of an entire, separate, pilot or control burner. Although passing reference is made to the possibility of applying the principles of Noir et al. '172 to other flame characteristics, such as the ionization of burned gases, no disclosure is provided or even remotely alluded to, as to how to practice such application.
Further, the Noir et al. reference is not directed to an apparatus suitable for use at widely varying firing rates. It would be desirable to provide a control apparatus having a method of control which could be provided at low cost, and capable of providing accurate burner control over a wide range of firing rates.
Problems faced by gas burners include performance variations caused by changes in air flow, due to fan/blower degradation and flue blockage, as well as changes in fuel heating value. Variations in burner performance caused by the aforementioned conditions can result in excessive pollutant production, which in turn can be a health and safety hazard. To compensate for these potential problems and provide a large margin of safety, current gas burner equipped appliances operate with a large amount of excess air. This large amount of excess air results in significantly lower system efficiencies.
An additional problem which gas burner equipped appliances, such as furnaces, face, is the effect which altitude has upon performance. At higher altitudes, burners receive air which is less dense, and accordingly, has less oxygen. Accordingly, for appliances which are not capable of modifying their operation in response to the altitude, such apparatus must be derated for altitudes which are different than a "base" or nominal optimum operating altitude (e.g., sea level). For example, it is typical to derate an appliance, such as a furnace, at a rate of -4% per every 1000 feet of increased altitude. That means, for an appliance having a rating of X BTU/hr at sea level, the rating will be X(1-0.04) BTU/hr at 1000 feet elevation.
It would be desirable to provide gas appliances with a way to self regulate, in response to changes in air flow, fuel heating value, and altitude so that substantially constant performance can be obtained, if desired (within the limits of the supply of available fuel and combustion air).